1. Field of the Invention
The present invention relates to an ink jet recording method and recording apparatus for forming characters and images on a recording medium by arranging ink droplets to adhere to the medium. Particularly, the invention relates to an ink jet recording method and recording apparatus suitable for performing a recording in high resolution or in high gradation. The recording method and recording apparatus of the present invention are applicable to all the equipment using a plain paper, a specially treated paper, a cloth, an OHP sheet, or the like. As suitable equipment, a printer, a copying machine, and a facsimile apparatus can be named specifically, among others.
2. Related Background Art
In recent years, personal computers, word processors, and other OA equipment are widely used. As a method of outputting on a recording medium the information entered by these kinds of equipment, there have been developed various recording methods, such types as wire-dot, thermal transfer, ink jet recording, for practical use. These recording methods are such that the formation of given images are made by each of the print heads on a recording sheet being fed in, but the kinds of recording heads are remarkably different from each other. Of these, the ink jet recording makes lesser noises, while making it possible to record in a higher quality and resolution, because there is no need for this type to be in contact with any recording media directly. In recent years, therefore, more attention has been given increasingly to this type of recording.
For a print head of an ink jet recording type, there is known a structure in which electrothermal transducing elements are arranged respectively for a sheet or each path of ink (nozzles) in which ink is retained. With this structure, driving signals are applied to the electrothermal transducing elements in accordance with recording information to cause them to generate the thermal energy that gives to ink surrounding them a rapid rise of temperature beyond nuclear boiling; hence creating film boiling on the thermoactive surface in each of nozzles of the print head. Then, an air bubble is formed in ink in the nozzle one to one by the corresponding driving signals thus provided. If the driving signals are provided in the form of pulses, the development and contraction of air bubbles are effectuated instantaneously. With this arrangement, therefore, it is possible to achieve the provision of a particularly quick response for the intended ink discharging.
As another structure of ink jet recording type, there is known a method in which electromechanical transducing elements (referred to as heaters) are arranged in or around each of ink paths, and then, a voltage is applied to each of the heaters in accordance with recording information. In this way, the mechanical pressure is exerted by the respective heaters to cause ink droplets to fly for the formation of images. Here, piezoelectric elements are known widely as the electromechanical transducing elements serving as means for generating discharging energy.
In this respect, as an invention that provides highly precise images by use of ink jet recording, a structure is disclosed in Japanese Patent Publication No. 54-41329. According to the structure disclosed in this publication, images (characters) formed at different dot pitches (resolutions) are recorded by dots of different sizes, large or small.
However, it is preferable to make the diameter of the printing dots small to form a unit pixel for the provision of high resolution to obtain high-quality images. This is because the unit pixel becomes smaller to form an image in this case, also because it is necessary to provide more printing dots if the resolution should be made higher on one and the same region in a recording area. For example, when a recording is performed in resolution of 360 dpi (dots per inch), a printing is executed by dots of 360.times.360 in a recording area of one square inch. However, should the printing be made in high resolution, such as in a density of 720.times.360 (dpi) or 720.times.720 (dpi), the number of recording dots is made larger by two or four times per unit area, respectively. In a usual recording, the driving frequency of a print head is set in a range where the performance of such recording is possible without generating any unstable discharges. Compared to a recording in usual resolution, high resolution recording requires dot numbers two to four times. Therefore, in order to execute high resolution recording without reducing the recording speed, it is necessary to increase the driving frequency of the print head two to four times. However, if the driving frequency is made higher than the expected capability of a print head, it may invite the generation of unstable discharges or an abnormal increase of temperature of a print head, which uses electrothermal transducing elements. As a result, the printing quality is lowered extremely. Therefore, it is impossible to increase the driving frequency higher than the expected capability of the recording head. This inevitably brings about a drawback that the printing should be made slower.
Also, in high resolution printing, the printing dot numbers becoming great, more energy should be applied to the printing head. The discharging energy at the time of ink discharge does not mean the total dissipation of the energy that has been given to discharging ink. Particularly when a type is adopted to discharge ink by the application of thermal energy using the electrothermal transducing elements described earlier, the residual energy is accumulated in the vicinity of the ink discharge ports of a print head. This excessive thermal energy causes the temperature of the printing head to rise after all. Meanwhile, the physical properties of ink used for recording, such as ink viscosity, are dependent on heat, and are directly related to the amount of the ink to be discharged. Therefore, the accumulation of the excessive heat on the print head produces adverse effects on controlling the discharging amount of ink; hence resulting in a drawback that the images are not recorded uniformly.
Also, when ink droplets of different sizes are discharged from one and the same nozzle, disturbance takes place in the stability of an intended discharge if the ink droplets of different sizes are being discharged mixedly in one and the same scan. This disturbance is caused by the fact that the ink jet type is provided with electrothermal transducing elements or the like for discharging ink in each of the ink paths, and that when ink droplets of different sizes are discharged, the ink flowing in and out of each ink path becomes uneven to hinder the stable ink discharging.
Also, whereas the life of a print head is essentially regulated by the number of ink discharging, the number of dots to be arranged in one and the same area is increased far more than the number required for low resolution printing when effectuating a highly densified printing; thus making the number of sheets recordable by one print head smaller when executing the highly densified printing. Therefore, in carrying on high density printing, the life of the print head tends to become shorter as compared to printing in low resolution. Then print heads should be replaced more frequently, which inevitably results in increased running costs.
As the best mode embodying the present invention, it is possible to record in a plurality of different resolutions in dot diameters of different sizes depending on recording media to be used. In this mode, however, lattice points tend to be offset by different resolutions, and there are some cases where even slight offset of dots becomes easily noticeable on a recorded image if the printing is made without any adjustment.
Also, in order to enhance the printing density, a double impacting mode is adopted so that the same dot is printed twice on one and the same position. In this mode, however, a portion recorded in high density is degraded by blurred ink in some cases if ink droplets of the same size are placed for recording in one and the same position.
Also, in the conventional art, it is found that no study has been made as to the status of resolutions switched over from one to another or the status of the kinds of images (dot sizes, for example) which are changed when recording is executed by use of dots of different sizes in different resolutions.
The present inventors have studied the situations brought by changes of resolutions and kinds of images, and found problems anew. In other words, by changes in dot sizes, that is, changes of one resolution to another, there occurs a problem that when high and low resolution recording areas are relatively close to each other, a portion where images are missing or an excessively densified portion is locally created in an ink image. Such portion is also created by the image itself that has been changed into being in high resolution.
Here is a specific example. Generally, along the adoption of high resolution intended in anticipation of obtaining a high-quality image, the unit picture element (unit pixel) that forms an image becomes smaller. Therefore, it is desirable to make the diameter of printing dots smaller in forming a unit pixel. This is because, unlike an electronic photography or a system that uses solid ink, the ink jet type that uses liquid ink requires reduction of the amount of ink adhering to a recording medium in order to avoid any excessive deposition of ink; hence preventing degradation of image quality on a recording medium, and also, deformation of the recording medium itself (that is, preventing it from presenting a waving condition called cockling) due to the excessive deposition of ink on it.
As described above, when dots are positioned on the lattice points arranged respectively in accordance with a plurality of different resolutions, the lattice points (the center points of arranged dots) are offset by each of the different resolutions applied, thus causing the arranged dots to be shifted. This situation is shown in FIG. 35. FIG. 35 illustrates an image formed by ink droplets discharged from a discharge port 23 by each of the scans 1 to 6 by the recording head 102. As clear from the relative positions of the recording head 102, which are designated by numerals 1 to 6 with respect to the recorded image, the recording medium and recording head are allowed to scan correlatively per scanning. In FIG. 35, the recording is performed with large dots at first in comparatively low resolution, and on the way, the recording is switched over to the one being carried out with small dots in high resolution. As clear from FIG. 35, the portion where an image is essentially missing occurs in an ink image as a gap A resulting from the dot shifting on the boundaries between each of areas in which large and small dots are arranged. In the gap A, there are changes in the resolution and size of dots to be recorded, which clearly indicates that the printing modes have been changed. In FIG. 35, white portions appear in the lattices as white streaks.
Also, in FIGS. 36A and 36B, an example is shown, in which different kinds of images (here, large dots and small dots) are mixedly present in one and the same luster. FIG. 36A illustrates the sizes and positions of recorded dots, while FIG. 36B represents the dots being plastered in order to make the recording result easily observable. It is understandable from FIGS. 36A and 36B that the gap B occurs in the junction between the large and small dots due to the displacement of lattice points in the dot arrangement. Also, it is observable that there partly appears the area C where the density is enhanced although extremely locally.
Now, as described above, a drawback is encountered in this respect that the uniformity of recorded image is disturbed when printing modes, such as resolutions, are changed according to the kinds of image to be recorded.
Further, for the enhancement of printing density, a double impacting mode is applicable. In this mode, dots are impacted twice on one and the same portion, but this mode also creates a problem following the changes of resolutions. With this mode, it is possible to attain the enhancement of density, but particularly when recording is performed in high resolution, the blurred ink may affect the portion that has been recorded in low resolution in some cases, and the image quality is degraded eventually.